1. Field of the Invention
The present invention relates to a zoom lens used for an objective lens of a camera and having a function of correcting undesirable displacement of a photographic image on a focal plane due to vibration of the camera.
2. Related Background Art
There have recently been developed various kinds of zoom lenses suited for compact cameras. In particular, such a zoom lens has been proposed that has a function of correcting undesirable displacement of a photographic image occurring by unintentional movement of the operator's hands when photographs are taken.
In general, a vibration correcting mechanism is designed to stabilize an image by moving one or several optical elements of an optical system in directions in which undesirable displacement of the image due to vibration is canceled. In a concrete vibration correcting mechanism, vibration correction can be attained by moving one or several lens groups of a zooming optical system perpendicularly to the optical axis. In such a mechanism, it is demanded that the structure of the lens groups moved for vibration correction be simplified.
For example, the vibration correcting mechanism disclosed in U.S. Pat. No. 5,040,881 has a zooming optical system comprising a first lens group, a zooming lens group and a group including a lens or lenses movable for focusing, arranged in this order as viewed from an object. Part of the first lens group constitutes a vibration correcting lens group and is moved perpendicularly to the optical axis so as to correct displacement of the image when the zooming optical lens system inclines.
Generally, the more corrected are aberrations of a vibration correcting lens group itself, the more is advantageously allowed the amount of vibration. When the vibration correcting lens group is eccentrically moved as a unit, aberrations are retained corrected well during its eccentric movement.
It is preferable, in principle, that the number of each lens group of a zoom lens system be as small as possible so that the zoom lens system has a simple structure. Thus, the lens system including a vibration correcting lens group would be constructed by a possibly smaller number of lenses. However, the number of lenses of the lens group must be increased in order to correct the aberrations of the vibration lens group well. In this case, the aberrations inherent in the lens groups including the vibration correcting groups, i.e., the aberrations allotted to the lens groups when they have been designed, must be properly produced in the lens groups other than the vibration lens groups. Thus, the number of lenses of the other lens groups is increased.
When, in particular, the first lens group is selected for vibration correction in a general photographic zoom lens system comprising three or more lens groups, the first lens group has a positive refracting power and tends to have a relatively large effective diameter. Thus, the lens group itself which must be moved as a unit becomes very heavy. The heavy weight deteriorates the response to the eccentric movement.
With the conventional zoom lens, therefore, it has been extremely difficult to satisfy both the requirements for high focusing ability and high response.